Bad to the Bone
by Get Keen
Summary: This is book about two stories. Yours, a human who fell into the world of Undertale, the world of which you knew and loved thanks to countless playthroughs of the game. And Sans, Sans the skeleton's. Well, his story after he absorbs your soul at any rate. Follow your own journey through a place you never dreamt existed and Sans' quest to break the barrier in this fanfiction.
1. Chapter 1

**Chapter 1**

Thick, red blood spattered the floor of the golden floor tiles of the sunlit hall. The body of a human lay half submerged in the crimson pool that grew ever greater around them. Their thick hair was matted to their head and darkened progressively as their life's source congealed and adhered to their skull. The gaping wound in their chest was where the dark liquid had first gushed, then seeped, and now merely trickled out of. On the other side of the majestic, stained glass windows birds could be heard chirping in the bright, cheerful light of the sun that was now streaming through into the hallway. It illuminated the dust particles that twinkled in the air like miniature stars. The rays of light also fell on the only other figure in the hallway, curled up into the foetal position and whimpering softly.

The figure was clothed in a thick winter jacket with a furry hood, black basketball shorts and a pair of furry, pink slippers. These clothes too, were spattered with the blood of the human lying a little ways away, as well as the figure's skeletal face.

"kid. i-i'm sorry. i'm so…sorry," the skeleton sobbed, tears that were not physically feasible rolling down his cheekbones. He tasted the unique metallic quality in his mouth and spat at the floor in disgust. He was disgusted at himself for being responsible for it being there, and not inside the fragile human body to which it belonged.

He got to his hands and knees and crawled weakly over to the human, crippled by an amalgamation of despair and guilt that vibrated through his bones.

"alright then, kid. you wanted to do this, so let's do it. just you 'n me." Sans swallowed the bile that was rising up in his throat. He couldn't believe the human had made him do this to them, had… _wanted_ this. He thought that somewhere, deep down, he should have felt honoured, or at the very least solemn assent at the human's insistence on _him_ being the one to do it. But he only felt regret.

He should never have agreed to this, but the human's reason behind the request was a noble one. Besides, he never could say 'no' to them.

New tears forming in the corners of his eye sockets, he held out his hand to the lifeless form in front of him. As it started to radiate with an aquamarine energy, a small red bundle of light rose out of the human's form. Their soul.

Sans shut his eyes and looked away, guilt racking his body when he saw the delicate cracks that had formed around the heart-shaped manifestation of all the human had ever been.

He took a deep breath and addressed the fragile red orb, "this is it kid. i'll see ya real soon."

Without giving himself the chance to change his mind, he tugged the heart closer to himself and whipped open his jacket, revealing his own pulsating blue-green soul. As if it knew exactly what it had to do, the human's red soul flew into the skeleton's rib cage and collided with his turquoise one. Sans doubled over, as the two souls fused together in what felt like a frantic dance inside of him. An inhuman wail ripped from his throat, his round, usually almost cartoon-like face twisted in pain. He dropped once again to the floor, his hands clutching at the fabric of the jacket around his chest.

Sickening grinding sounds echoed along the hallway as the skeleton's body began changing, to accommodate the extra soul that was now inside of him. Although Sans had known that it was going to happen, nothing could have prepared him for the mind-numbing pain that accompanied the transformation.

The bones that made up his limbs lengthened significantly, the joints cracking and popping as they did so, drawing sounds out of Sans that he had never thought he would ever have been capable of making. A sudden crack in his spine caused the skeleton to let out a bestial hiss and he arched his back into the air. His shoulders broadened and his rib cage rose and expanded, every popping and crackling of a joint bringing pitiful whimpers to his teeth.

And then…nothing. The sudden transformations that had arrested Sans had stopped as unexpectedly as they had started.

The transfigured skeleton was left, panting and sweating on the floor, for a length of time he could not determine. When he did eventually attempt to rise from the tiled floor, he did so shakily and ashamed. The true monstrosity of what he had done was going to be reflected by himself, by his _own body_ , for the rest of his life.

"good," he said out loud at this thought, and was surprised to find that he still sounded like himself.

He bent over the human's body, almost falling over as he did so. His centre of gravity was thrown way off by his new found height. But still he struggled on, curling the arms that were now too long for his jacket gently around the lifeless form. Sans and the human had said they were going to finish what they had started together, and that was damn well what the skeleton was going to make sure happened. He lifted them to his chest, hearing a rip in his jacket as he did so and ignoring it. Staring at their bloodied figure, Sans felt his eyes prick once more, the threat of tears looming ahead. He let them come and a faintly glowing liquid trickled down his face, tracing its new, more fearsome contours.

He walked slowly along the corridor, the sunlight still beaming through invasively. He stopped and glared at the window that let through the traitorous sun that dared to show its face when everything around him was so dark.

What stared back at him from the stained glass was enough to make even Sans gasp and stumble backwards, clasping the human's body protectively to him. The white pinpricks of light everyone had come to recognise as Sans' "eyes" were glowing from eye sockets in a reflection that the skeleton didn't want to believe belonged to him. The slanted, almond-shaped nature of the sockets looked menacing and animalistic, despite the fact that the shape of Sans' skull hadn't appeared to have changed at all. His teeth now had the addition of sinister looking fangs, appearing all the more evil-looking due to his permanent smile. Two deep hollows ran along the length of his cheekbones in flat, black curved lines and, just before he managed to tear his eyes away, Sans saw the new, pointed ridge along his collar bone rising up to his jawline like the collar of a cape.

"i-i…i'm a _monster_ ", the last word was hissed in fury through gritted teeth and the tell-tale glow of Sans' left eye socket started up again.

But before he could do anything, he felt a little tug at his torso. He looked down in surprise, the energy in his eye quickly fizzling out. The human hadn't moved, obviously, but the tug had come from inside his chest cavity. Sans could feel their _soul_ pulling at his. It was giving him what felt like a nudge of gentle indignation. They seemed to disagree with his statement.

He continued to stare down at his chest in disbelief, the smile falling away from his face.

He felt it again. This time it literally felt like the human's soul was giving his a _hug_.

The human was trying to comfort him.

Tears once again built up in Sans' eye sockets and he felt heat rise to where his cheeks would have been, but this time he cried with relief. His smile returned and, for the first time in a long time, it was a genuine one.

Pulling away from the window decisively, Sans marched along the hallway, dappled in the sunlight that had now become tinged with red. His somehow still slippered feet swished against the tiles and, when they came to the door leading to the king's chambers, they made a muffled thud against the burgundy wood as he kicked it open.

Unexpectedly, there was no sign of Asgore anywhere. Sans and the golden flowers were the only living things in the room. As gently as he could, the monster placed the human's body on a bed of flowers in the centre of the floor where it sank into the soft petals.

He knelt down and placed a long, skeletal hand on the human's forehead, flinching at how cold it was already. With one more regretful look at their face, Sans reached out and closed their eyes with two phalanges before abruptly standing and turning on his heel.

He hesitated at the open doorway for a brief moment before walking out and closing it quietly behind him. He addressed his rib cage, still covered by the torso of his jacket. The arms had been ripped off, but the majority of the cloth and the furry hood still remained.

"we're seein' this through kid. but afterwards, i'm gonna find a way to get you your body back." The warmth that flooded his chest was almost overwhelming, but it gave Sans his answer.

"heh. ok kid. o.k." he whispered this last part more to himself, his brow furrowed with steely resolve. He marched down the corridor in the opposite direction he had just come. Asgore was the only one who could access the Barrier and that was precisely where Sans wanted – no, _needed_ – to be. He just needed to find the inconveniently absent king first.

Sans broke through into the intense heat of Hotland, but even the climate that he had always hated could not break his newly regained sense of purpose.

This may have just been due to the newest addition of the human's soul to his body but, for the first time in his life, Sans was filled with **determination.**


	2. Chapter 2

**Chapter 2**

Author's note: Thank you to everyone who has already given my little book so much support! It means the world to me! To _Kiryma_ and everyone else like them who thought the description of his facial transformation was not clear enough (my sincere apologies for that) Sans now has a ridge along his collar bone around his neck. Picture an old fashioned 'Dracula' cape with one of those sleek collars. Thanks once again everyone!

Sincerely yours, Get Keen.

"…Please do not return. You understand."

You sniffle in the ethereal white-blue glow from your computer screen as Toriel bends down and hugs Frisk. A half-full box of Kleenex sits on the desk next to your whirring laptop, the other half of its contents strewn about the glass surface. You grope for another tissue while your eyes stay fixed on the screen, your chin resting in your other hand as you watch Toriel leave the room.

"Tori, no!" You wail, drawing out the 'o' sound for an unnecessarily long time, "I'm sorry! Come back!" You had just started _another_ pacifist playthrough of Undertale. Although you'd watched other people play the genocide run, which was a traumatic enough experience, you could never bring yourself to even begin it on your own laptop. And so, you had contented yourself with multiple versions of the pacifist gameplay, freeing the monsters from the Underground time and time again and sparing anyone and everyone who came up against you.

It gave you a warm, fuzzy feeling inside. A sense of purpose.

You yawn unceremoniously, minimising the game to check the built in clock on your computer. 2:56 am.

"Wowie," you sigh and then chuckle at yourself. You had obviously been playing this game way too much, the characters' dialects were starting to rub off on you. You groan as you get up reluctantly from your hunched position over the computer, slapping the laptop shut without switching it off. You grab the red fleece blanket that had been hanging over the back of your desk chair and cocoon yourself in it, pulling the fluffy material to your chin, before collapsing onto your bed.

A chill permeates the room through a window that is still open and you glare at it tiredly. After a few tense moments of a stare down with the blowing curtains, you turn onto your side and snuggle further into the little pocket of warmth you'd created for yourself, falling asleep to the Undertale soundtrack still drifting through your mind.

It is a deep, penetrating coldness that drags you from your slumber. Shivering and mentally cursing your own laziness at not closing the window, you grope around you for the blanket that seemed to have unwound itself from your body during the night. Your hand instead plunges into a wet, mushy substance. And man, is it _cold._ A breathless gasp rises from your lungs and your eyes spring open.

And what a sight greets that disoriented gaze.

You are laying on the edge of a paved path that is lined with pristine, powdery snow, spreading into a dense forest of colossal pine trees. Your unfortunate hand had, instead of the warm cover you had been seeking, found an icy mound next to the hard stones of the path. Intricate snowflakes drifting in a gentle, winter breeze constantly add to the sleek white coat the pines had attained, as well as the seemingly endless fields of snow that disappear into the thick shadows of the forest bordering them.

You feel your jaw drop.

The place would be beautiful, if it weren't for its mildly eerie atmosphere. You get up from the ground, arms wrapped around your body, and consider dismissing the sudden change of scenery merely as a vivid dream. However, the bitter coldness of the snow your unfortunate hand had discovered was too harsh to be false.

This was no dream. The realisation chills you even further, and you wrap your arms around yourself for comfort. Instead of the cloth of your pyjamas though, your cold fingers clutch onto thick, soft wool. You look down in surprise and find yourself not in the simple shirt and shorts you usually sleep in, but fully kitted out in black ankle boots, faded jeans and an awfully familiar woollen jumper.

You do a double take. It couldn't be…could it?

The notion is absurd even to you, but what else could explain your frosty surroundings and new purple-and-blue striped jumper?

You seemed to be, unbelievably, in Undertale.

A sudden thought strikes you and you paw at your face frantically, making sure that everything is the same as it had been when you had went to bed the night before. You heave a sigh of relief as you poke the tip of your nose, discovering that the entirety is as it should be. You hadn't physically turned into Frisk, despite the fact that you were wearing their unmistakable clothing.

You take a deep breath and take a few stiff steps forward, thoughts whizzing through your mind. After a while your steps, therapeutic in their regularity, calm you down, your initial panic dissolves and your scrambled thoughts untwine and actually begin to make sense. You had just started another game right? And what better way to play through it than with a totally _immersive_ experience. You chuckle weakly and start to brighten up. Ever since you started playing the game, you had loved this virtual world and all of its unique, quirky characters. And apparently, you had the chance to actually live through it yourself. Now that you think about it, this is actually brilliant! A bounce finds its way into your step and you saunter along the path, the cold no longer bothering you so much.

You carry on walking with nothing breaking the monotony of the snow and trees, until you come across a long, thin stick. It was so out of place, it was almost ridiculous. But you remember this particular wooden landmark and you step over it purposefully, a knowing smile finding its way onto your face. You clasp your hands behind your back as you walk on, whistling Megalovania nonchalantly. When you hear the quiet crackle of wood splintering, you whip around to see the now broken stick. There was no sign of its breaker anywhere, however.

You turn on your heel and hurry on along the path, this was getting interesting and you couldn't wait to see what would happen next. The fact you had already played through this scenario countless times was evidently lost on you in your child-like excitement.

When you see a short fence with gaps too wide to actually function as a barrier, you screech to a halt and take a moment to gather your thoughts before you continue.

"Wow, this really _is_ happening," you pant, breathless and bright-eyed with anticipation. You take a couple of deep breaths to slow your heart rate before taking a measured walk up to the fence. After all, you were convinced that somewhere a short, lazy skeleton was watching your every move. This was obviously the case as you had scarcely reached the fence when you heard muffled footsteps on the path behind you.

" **human** ," a thrill runs up and down your spine and you fill your lungs with biting air as you let out an involuntary gasp.

" **don't you know how to greet a new pal?** " you almost burst a blood vessel trying to keep in squeals of delight.

" **turn around. . .** "

You look down at the frosted floor and turn around slowly. Out of the corner of your eye, you see movement, an outstretched arm. You hold out your own hand and slowly raise your eyes to the figure standing in front of you.

As the distinctive sound of a whoopee cushion fills the air and makes you cringe, you gaze star-struck a character you had never dared to wish could be real.

"i'm sans. sans the skeleton."


	3. Chapter 3

**Chapter 3**

Sweat dripped from Sans' brow in the heat of his surroundings as he summoned up one of his 'shortcuts'. He raised a hand to his face in order to wipe some of the moisture off and ended up clobbering himself in the skull, not yet used to his arm's new, increased length. He glared at his limb, feeling a little betrayed, before stepping into the gap in the space-time continuum he had casually created.

When he emerged on the other side, the skeleton sighed with happiness as he felt the familiar, if somewhat dusty carpet underneath his slippered feet. He never _had_ been the best at cleaning his room. The cavity behind him was still letting in heat from the lava land he'd just left behind and Sans closed it in irritation. He fell backwards onto his bed, folded both arms under his head and stared up at the ceiling. He stayed like that, quietly reliving the traumatising events of the past couple of hours until a demanding knock on his door yanked him out of his pit of deep, somewhat dark, thoughts.

Sans sat bolt upright on his mattress and stared at the door, frozen in horror. He couldn't let anyone know what he'd done, not until he'd got the human their body back…somehow. But Sans was frozen in place, he could only stare transfixed at the doorknob as it began to turn slowly. It was the loud click the door made signalling that it was about to open that startled Sans into action. He grabbed his balled up blanket on the mattress beside him and shook it out frantically. Tucking his long legs underneath him, he tried to make himself appear as small as possible as he spread the blanket over his body.

He had just managed to pull it over his head when he heard loud footsteps stomp across the room to where he lay.

"Get up, lazybones! We have a big day ahead of us! The human said they were going to break the barrier remember? We have to be there to support our _friend_ ," Sans' brother Papyrus was tugging at the blanket. He said the last word emphatically, his very first human friend was extremely important to him and he wanted to show them that he cared. Sans grunted in reply, too terrified to form actual words.

"Sans! Come _on_!" the tall skeleton pulled harder, only making his brother hold on tighter. Sans' eye was starting to glow in panic under the blanket. Papyrus, his baby brother, was the last person he wanted to see him like this. All his life he had protected him with everything he had, and now…now he was the very thing Papyrus needed protecting _from_. He felt the cover start to give way and he clutched onto it tighter, his eye burning brighter with desperate aquamarine energy.

"I thought you liked the human Sans," Papyrus, still pulling on the blanket with all his might, decided to change tack, "But obviously you don't care about them enough to go and support them." This was a mistake.

The build-up of emotions churning inside the skeleton had reached a boiling point and at these words, something inside Sans snapped. His eye flamed with power and the blanket, as well as everything else in the room, glowed turquoise and rose into the air. Sans leapt out of the bed and towered over his normally taller brother who scrambled backwards and pressed himself up against a wall at the sight before him. Sans, vision blurred in his fury, approached the trembling skeleton who was staring up at him in terror. When he reached him, he bent over and pressed his fearsome face against Papyrus'.

 **"** **i care about the human more than you could ever know."** the grin on the monster's face grew sinister in his anger. A long stretch of tense silence permeated the room as Sans breathed angrily onto his brother's terrified face. Papyrus studied the other skeleton carefully and tears filled his eye sockets as he took in every facet of Sans' terrible transformation. He knew that his big brother must have committed something unthinkable to make him so monstrous. He could hardly reconcile the looming skeleton in front of him with his short, lazy, wise-cracking brother.

Papyrus' knees shook and his voice quivered as he replied, "S-Sans, just know th-that whatever you may have done…I s-still believe in you."

With that one innocent, trusting statement all the fury drained out of Sans' body and the glow left his eye. His vision gradually became clearer and, when he saw the tears running down Papyrus' face he slumped to the floor. Racked with guilt, Sans curled into himself and shuddered in utter desolation. He had been an absolute _beast_ to his own baby brother. He wouldn't be surprised if Papyrus never wanted to lay eyes on him again.

So you can imagine his shock when he felt a gentle hand on his back, rubbing small circles into his shoulder blades. He gasped, remembering that this was precisely what he had done to calm down Papyrus when he was just a baby bones. He looked over his shoulder hesitantly and found his brother smiling sadly down at him.

"Do you remember when I had that nightmare, brother?" he said softly, the soothing circular movements continuing. Sans nodded wordlessly. Papyrus had only ever had one nightmare in his life and he had never spoken a word about it. As if reading his thoughts, Papyrus said, "I never did tell you what it was about, did I?" Sans shook his head and slowly got into a sitting position, cross-legged on the floor facing Papyrus. The younger skeleton now took his hand and started to rub even smaller circles into his palm. Sans watched his movements ashamedly. He didn't deserve this.

Papyrus took a deep breath, "It was about losing you, Sans. My nightmare was that I didn't have you around at all, to protect me, to tell me bedtime stories," they both smiled slightly at that, "or to comfort me when I was upset." Papyrus let his older brother's hand go now and Sans looked involuntarily up into his eyes. "You should know, brother, that this is still my biggest fear…" Papyrus said quietly. Sans spoke now, voice thick with emotion, "but even after i -".

Papyrus cut him off, "I _said_ , that is _still my biggest fear, Sans._ " Sans only stared at the other skeleton in shock before a tentative smile lit up his face. Without warning, he threw his long arms around Papyrus' shoulders. The younger skeleton hugged him back tightly and pressed his face into his brother's jacket, breathing in its familiar smell. He may have changed physically, but Papyrus knew Sans was still the same on the inside, and that was what mattered.

"I-I love you, big brother." Sans sobbed into his brother's red scarf at these words, and hugged him tighter. "No matter what," Papyrus wheezed, tapping Sans on the arm to let him know that he was running out of oxygen. Chuckling, the larger skeleton obliged reluctantly and they both got to their feet.

Sans rubbed the back of his head sheepishly, "heh. sorry, bro. i guess you could say the feels were too **strong**." The pun was so unexpected and downright _absurd_ coming from his brother's frightening figure that Papyrus actually laughed, and a radiant smile graced Sans' face as a result. The younger skeleton beamed back at him and decided that his brother didn't look so terrifying after all and, if he needed to, the Great Papyrus would show everyone that.

Heading to the door, he yelled back over his shoulder, "I will fix up some spaghetti for us both and then you, brother, can tell me exactly what happened to make you," he hesitated, "er, this way." Sans shuffled after his brother obediently, deciding that Papyrus deserved to know the truth. When Sans reached the landing, the other skeleton was already marching towards the kitchen but before he disappeared through the doorway, a thought struck Sans and he called out to him.

"wait!" Papyrus stuck his head out and looked at his brother quizzically. Sans scuffed his foot against the wooden floor and blushed deeply before continuing.

"i…love you too, pap."


End file.
